Thermistor control circuit

ABSTRACT

An indirectly heated thermistor control circuit wherein a constant current source is connected with the bead resistance of the thermistor to provide a reference voltage proportional to variations in bead resistance. The reference voltage is connected by a feedback path to the non-inverting input of a differential amplifier where it is combined with the input command voltage which is applied to the inverting input of the differential amplifier. The output signal from the differential amplifier provides the thermistor heater current and is proportional to the difference between the reference voltage and command voltage.

[72] Inventor:

United States Patent Tentarelli [54] THERMISTOR CONTROL CIRCUIT KennethD. Tentarelli, Atkinson, N.l-l.

Bell Telephone Laboratories, Incorporated, Murray Hill, BerkeleyHeights, NJ.

22 Filed: Jan. 19, 1970 21 Appl.No.: 3,989

[73] Assignee:

[52] US. Cl ..323/8, 323/68 [51] Int. Cl. ..G05l UN [58] Field of Search..323/68, 69, 21, 8; 338/23,

[56] References Cited UNlTED STATES. PATENTS 3,462,615 8/1969 Bernstein..307/311 X 3,479,559 11/1969 Paget 323/21 UX 3,441,892 4/1969Luedeman.... ..338/23 3,466,572 9/1969 Hanna et a1. ..323/21 UX 1 June6,1972

OTHER PUBLICATIONS IBM Technical Disclosure Bulletin MultichannelConstant Current Supply" Vol. 11, No. 2, page 1 10, 7-68.

Primary xaminerRodney D. Bennett, Jr. Assistant Examiner-R. KinbergAnorney--R. J. Guenther and E. W. Adams, Jr.

[57] ABSTRACT An indirectly heated thermistor control circuit wherein aconstant current source is connected with the bead resistance of thethermistor to provide a reference voltage proportional to variations inbead resistance. The reference voltage is connected by afeedback path tothe non-inverting input of a differential amplifier where it is combinedwith the input command voltage which is applied to the inverting inputof the differential amplifier. The output signal from the differentialamplifier provides the thermistor heater current and is proportionalto-the difference between the reference voltage and command voltage.

6 Claims, 2 Drawing Figures TO OTHER CONTROL UNITS PATENTED un 6 m2 FIG.8

CONSTANT CURRENT 1|- SOURCE e E F g TOOTHER CONTROL UNITS FIG. 2

c 5 G 10 i 4 EC V \H To OTHER 5 CONTROL UNITS lNl/ENTOR B K. 0. TENTARELL/ ATTORNEY TI-IERIVIISTOR CONTROL CIRCUIT BACKGROUND OF THEINVENTION Since the discovery of the variable resistance properties ofcertain semiconductor material, indirectly heated thermistors,

i.e., thermistors which have a heater element,have been used as controlelements. Although the use of themiistors is widespread, certaininherent deficiencies of indirectly heated thermistors have plaguedcircuit designers. These deficiencies include the errors introduced bythe non-linear dependence of thermistor bead resistance on heatercurrent and the relatively large manufacturing tolerances ofthermistors. Since the heater current is usually controlled so as toaffect the bead resistance and thereby introduce the desired control,the nonlinear dependence of thermistor bead resistance on heater currentmakes it extremely difficult to obtain precise tracking in open loopregulating repeaters and equalizers, hereinafter referred to asregulators, employed in modern telephone systems. Similarly, typicalthermistor manufacturing tolerances are intolerable in precision openloop regulators without compensatory circuit adjustment, and tend toreduce the available dynamic range in closed-loop regulators. Tightermanufacturing tolerances help overcome this latter deficiency but aging.can cause several percent bead resistance shift over the first fewyears of use of the thermistor.

Other deficiencies which introduce difficulties in the use of indirectlyheated thermistors in control circuitry are environment temperaturesensitivity and high frequency heating. Since control of an indirectlyheated thermistor is effected thermally, changes in bead resistance dueto environment temperature rather than heater current results in systemerror in open loop regulators and loss of dynamic range in closedloopregulators. In prior art circuits, thermistors are often placed inconstant temperature housings or ovens to avoid these effects ofenvironment temperature. High frequency signal currents vary with systemlevels and loading and have an effect on thermistor regulatorperformance similar to variations in ambient temperature.

In view of these deficiencies, a number of attempts have been made toreplace indirectly heated thermistors in control circuits withrelatively new devices including Hall-effect devices and opticallycoupled lamp-photocell packages, but none of these allow the performancedemanded by the stringent systems requirements of modern telephonecommunication systems. Other attempts at using two matched themiistorsin place of a single thermistor in an attempt to offset the aforenoteddeficiencies have met with success only .over a relatively limiteddynamic operating range.

It is, therefore, an object of this invention to provide an indirectlyheated thermistor control circuit which overcomes these aforenoteddeficiencies.

It is a further object of the invention to provide an indirectly heatedthermistor control circuit wherein the bead resistance of the indirectlyheated thermistor is maintained proportional to the input commandvoltage.

SUMMARY OF THE INVENTION In the present invention, a differentialamplifier having an inverting and a non-inverting input is combined witha constant current source to control directly the bead resistance of anindirectly heated thermistor in accordance with a command signal. Thecommand signal is connected to the inverting input of the differentialamplifier with the output of the amplifier connected to the heatingelement of the indirectly heated thermistor. The bead resistance of thethermistor, which is connected to the output circuitry to be controlled,is also connected to a constant current source to establish a referencevoltage which is proportional to the bead resistance.

A feedback loop connects this reference voltage to the non-invertinginput of the differential amplifier. Since the output current of theamplifier determines the heat generated by the thermistor heater elementand this current is directly proportional to thedifference between thebead resistance reference voltage and the command voltage, directcompensation is provided for variations in the bead resistance referencevoltage. For example, if the bead resistance of the thermistor were toincrease due to a decrease in environment temperature, then thereference voltage would increase by the same magnitude, the difierencebetween the reference voltage and the command voltage would increase,and the heater current would increase, thereby characteristicallydecreasing the bead resistance and compensating for the initialincrease. Each of the aforenoted deficiencies of indirectly heatedthermistor circuits is compensated in a like manner as discussed indetail hereinafter. The bead resistance is thus directly proportional tothe command voltage and substantially independent of all otherparameters. As an added advantage, the present invention permits severalthermistor control circuits to be driven from a single source of commandvoltage. Semiconductor devices which suffer from one or more of thedeficiencies noted in connection with thermistors, e.g., field-effecttransistors, may also be employed in the practice of the presentinvention; as discussed hereinafter.

BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION As noted, FIG. 1of the drawing is a simplified embodiment of the present invention. Inthe circuit of FIG. 1, amplifier 1 is a high gain trans conductanceamplifier having an inverting 2 and a non-inverting 3 differentialinputs. The heating element 4 of the control thermistor 5 is connectedbetween the output of the amplifier l and ground. The bead resistance 6of thermistor 5 is connected to the output terminals 7 of the controlnetwork. A constant current source 8 is serially connected with the beadresistance 6 and ground. A feedback path is connected from the junctionof the constant current source 8 and the bead resistance 6 to thenon-inverting input to the amplifier 1. An input or command voltage isconnected between ground and the inverting input 2 of the amplifier 1.

In the thermistor control circuit of FIG. I, it is desired to vary thebead resistance 6 linearly with the command voltage E, appearing at theinverting input 2 of the amplifier I. In other words, the outputamplifier current 1,, should vary the bead resistance 6 of thermistor 5linearly with changes in command voltage and independently of thedeficiencies of indirectly heated thennistors noted heretofore.Circuitry responsive to the resistance of head resistance 6 would beconnected to the output terminals to be controlled directly by thecommand voltage E,.. For example, the present indirectly heatedthermistor control circuit might be employed in a system havingregulating repeaters and equalizers similar to those of the L4 CoaxialCarrier System disclosed in detail in the April 1969 issue of The BellSystem Technical Journal. In such a system, the bead resistance ofindirectly heated thermistors is used in some places to control the gainof amplifiers.

In the present invention, constant current source 8 provides a constantcurrent reference used to generate a voltage E,, which is directlyproportional to the thermistor bead resistance R The reference voltageappears in turn at the noninverting input 3 of the amplifier 1 via thefeedback path. The instantaneous input e to the amplifier 1 may thus betreated as the difference between the reference voltage E, and thecommand voltage E If the transconductance gain of the amplifier issymbolized as G, as shown in the drawing, the heater current 1,, of theoutput of the amplifier may be expressed as follows:

It G(e)= G(E,,Ep) for values of e greater than zero. (The gain G of theamplifier is chosen so that 1,. will be zero for values of e less thanzero.) Since the bead resistance of an indirectly heated thermistor ischaracteristically inversely proportional to the heater current In,variations in 5,, due to deficiencies in the thermistor cause thevoltage 2 to vary in a manner which varies 1 and compensates for thethermistor variations or deficiencies. For example, if the resistance ofthe. bead resistance 6 were to increase due to a decrease in theenvironmental temperature, the reference voltage E1, would increase bythe same magnitude, the current output from the constant currentsourcebeing constant. From the foregoing expression for the heater currentI,,, it is seen that In would proportionally increase thereby increasingthe temperature of the heater 4 and decreasing the resistance of thebead resistance 6. The'initial variation of increased bead resistance 6is thus compensated and the bead resistance 6 is thereby proportional tothe command voltage E Variations in bead resistance due to highfrequency heating and manufacturing tolerances are compensated for in asimilar manner. Since with the present invention bead resistance is adirectly controlled variable, the non-linearity of heater current vs.bead resistance has no affect on circuit response. Each of the noteddeficiencies of indirectly heated thermistor control circuits is thusovercome by the present invention. Since the bead resistance isinversely related to the heater current the feedback loop may becharacterized as a negative feedback 100 A still further advantage ofthe present invention is that the isolation andhigh gain presented bythe amplifier 1 enables other similar indirectly heated thermistorcontrol units to be driven by the same command voltage. Precise,identical control of multiple thermistor units is thus possible.

A more detailed embodiment of the present invention isillustrated inFIG. 2. In the circuit of FIG. 2, the components identified in FIG. Ibear the same numerical designations. In FIG. 2, a transistor has itsbase electrode connected to the output of the amplifier 1. The emitterelectrode of transistor 10 is connected to ground through currentlimiting resistor .1 1. Resistor 12 connects the collector electrode oftransistor 10 with the heater 4 of thermistor 5 and a source of biasingpotential, symbolically represented as a battery, which is alsoconnected to ground. An inductor 13 is serially connected with the beadresistance 6 of thermistor 5, inductor 14, constant current source 8,and ground. The constant current source 8 is schematically representedas a conventional onetransistor regulator with a source of constantbiasing potential. Capacitor is connected from the junction of inductorl3 and bead resistance 6 to one of the output terminals 7, whilecapacitor 16 is connected from the junction of inductor l4 and beadresistance 6 to the other output terminal 7.

Transistor 10 serves as a driver stage and enables the differentialamplifier l, which has a relatively high gain, to be fabricated usingintegrated circuit techniques. Inductors 13 and 14 serve as RF chokeswhich block high frequency signals from equipments connected to theoutput terminals 7 from interfering with the operation of the circuit ofFIG. 2. Similarly, capacitors l5 and 16 serve asDC blocking capacitors.The operation of the circuit of FIG. 2 is the same as that of thecircuit of FIG. 1 with the gain of transistor 10 detemtined by therequired heater current of thermistor 5.

Although the foregoing discussion is directed primarily to a particularsemiconductor device, the thermistor, it should be obvious that othersemiconductor devices which suffer from one or more of the deficienciesnoted heretofore in connection with thermistors, could also be employedin the practice of the present invention. For example, over the portionof their characteristic that they are employed as variable resistors,field-effect transistors exhibit a non-linear characteristic similar tothat of thermistors. In the circuit of FIG. 1, the source electrode ofthe field-effect transistor would be connected to ground, the gateelectrode would be connected to the output of the amplifier l, and thedrain electrode would be connected to the constant current source 8 andthe feedback path to the input 3 of amplifier 1. Compensation for thenon-linearity would then be provided in the manner discussed heretofore.

The above-described arrangement is illustrative of the application ofthe principles of the invention. Other embodiments may be devised bythose skilled in the art without departing from the spirit and scopethereof.

What is claimed is:

1. A control circuit comprising an amplifier having inverting andnon-inverting inputs, a source of command voltage connected to saidinverting input of said amplifier, a semiconductor device having inputand output temiinals, means connecting the output of said amplifier tothe input terminals of said semiconductor device to control theimpedance presented by the output terminals of said semiconductor devicein accordance with said command voltage, an output circuit to becontrolled in accordance with the impedance presented at the outputterminals of said semiconductor device connected to the output terminalsof said semiconductor device, a constant current source seriallyconnected with the output terminals of said semiconductor device toprovide a reference voltage proportional to the impedance presented atthe output terminals of said semiconductor device, and feedback meansconnecting said reference voltage from the output terminals of saidsemiconductor device to the non-inverting input of said amplifier, saidoutput circuit and said reference feedback voltage being therebycontrolled in accordance with the single impedance presented at theoutput terminals of said semiconductor device.

2. An indirectly heated thermistor control circuit comprising anamplifier having inverting and non-inverting inputs, a source of commandvoltage connected to said inverting input of said amplifier, anindirectly heated thermistor having its heater connected to the outputof said amplifier, the bead resistance of said thermistor beingconnected to an output circuit which is controlled in accordance withsaid bead resistance, a constant current source serially connected withsaid bead resistance of said thermistor to provide a reference voltageacross said bead resistance which is proportional to said beadresistance, and feedback means connecting said reference voltage fromsaid bead resistance to the noninverting input to said amplifier tosupply said reference voltage to said amplifier, said output circuit andsaid reference voltage being both controlled in accordance with saidsingle bead resistance.

3. An indirectly heated thermistor control circuit comprising anamplifier having inverting and non-inverting inputs, a source of inputcommand voltage connected to said inverting input of said amplifier, adriving transistor having its base electrode connected to the output ofsaid amplifier, the emitter and collector electrodes of said drivingtransistor being connected with the heater of an indirectly heatedthermistor and a source of biasing potential, the bead resistance ofsaid indirectly heated thermistor being connected to an output circuitwhich is controlled in accordance with said bead resistance, a constantcurrent source serially connected with said head resistance of saidthermistor to provide a reference voltage across said bead resistancewhich is proportional to said bead resistance, and feedback meansconnecting said reference voltage from said bead resistance to thenon-inverting input to said amplifier to supply said reference voltageto said amplifier, said output circuit and said reference voltage beingboth controlled in accordance with said single bead resistance, wherebythe thermistor bead resistance is maintained directly proportional tothe said input command voltage and is independent of thermistordeficiencies.

4. An indirectly heated thennistor control circuit in accordance withclaim 3 wherein said amplifier circuit is an integrated circuit.

5. An indirectly heated thermistor control circuit in accordance withclaim 3 wherein a first inductor is connected between one temiinal ofsaid bead resistance and one terminal of said constant current source,and a second inductor is serially connected between the other terminalof said head resistance and the other terminal of said constant currentsource.

1. A control circuit comprising an amplifier having inverting andnon-inverting inputs, a source of command voltage connected to saidinverting input of said amplifier, a semiconductor device having inputand output terminals, means connecting the output of said amplifier tothe input terminals of said semiconductor device to control theimpedance presented by the output terminals of said semiconductor devicein accordance with said command voltage, an output circuit to becontrolled in accordance with the impedance presented at the outputterminals of said semiconductor device connected to the output terminalsof said semiconductor device, a constant current source seriallyconnected with the output terminals of said semiconductor device toprovide a reference voltage proportional to the impedance presented atthe output terminals of said semiconductor device, and feedback meansconnecting said reference voltage from the output terminals of saidsemiconductor device to the noninverting input of said amplifier, saidoutput circuit and said reference feedback voltage being therebycontrolled in accordance with the single impedance presented at theoutput terminals of said semiconductor device.
 2. An indirectly heatedthermistor control circuit comprising an amplifIer having inverting andnon-inverting inputs, a source of command voltage connected to saidinverting input of said amplifier, an indirectly heated thermistorhaving its heater connected to the output of said amplifier, the beadresistance of said thermistor being connected to an output circuit whichis controlled in accordance with said bead resistance, a constantcurrent source serially connected with said bead resistance of saidthermistor to provide a reference voltage across said bead resistancewhich is proportional to said bead resistance, and feedback meansconnecting said reference voltage from said bead resistance to thenon-inverting input to said amplifier to supply said reference voltageto said amplifier, said output circuit and said reference voltage beingboth controlled in accordance with said single bead resistance.
 3. Anindirectly heated thermistor control circuit comprising an amplifierhaving inverting and non-inverting inputs, a source of input commandvoltage connected to said inverting input of said amplifier, a drivingtransistor having its base electrode connected to the output of saidamplifier, the emitter and collector electrodes of said drivingtransistor being connected with the heater of an indirectly heatedthermistor and a source of biasing potential, the bead resistance ofsaid indirectly heated thermistor being connected to an output circuitwhich is controlled in accordance with said bead resistance, a constantcurrent source serially connected with said bead resistance of saidthermistor to provide a reference voltage across said bead resistancewhich is proportional to said bead resistance, and feedback meansconnecting said reference voltage from said bead resistance to thenon-inverting input to said amplifier to supply said reference voltageto said amplifier, said output circuit and said reference voltage beingboth controlled in accordance with said single bead resistance, wherebythe thermistor bead resistance is maintained directly proportional tothe said input command voltage and is independent of thermistordeficiencies.
 4. An indirectly heated thermistor control circuit inaccordance with claim 3 wherein said amplifier circuit is an integratedcircuit.
 5. An indirectly heated thermistor control circuit inaccordance with claim 3 wherein a first inductor is connected betweenone terminal of said bead resistance and one terminal of said constantcurrent source, and a second inductor is serially connected between theother terminal of said bead resistance and the other terminal of saidconstant current source.
 6. An indirectly heated thermistor controlcircuit in accordance with claim 5 wherein a first capacitor isconnected between said one terminal of said bead resistance and oneterminal of said load circuit, and a second capacitor is connectedbetween said other terminal of said bead resistance and the otherterminal of said output circuit.